[Package Importer] Add types and specs

accepted/package-importer.changes.md

@@ -2,6 +2,9 @@
 
 * Change from an entry point path to entry point directory.
 
+* Add a private readonly symbol key to `NodePackageImporter` type to
+  distinguish it from arbitrary objects.
+
 ## Draft 1.5
 
 * Specify that Package Importers must not reject URL patterns that other Package

accepted/package-importer.d.ts.md

@@ -235,7 +235,12 @@ import {FileImporter, Importer} from '../spec/js-api/importer';
 ### `NodePackageImporter`
 
 ```ts
+declare const nodePackageImporterKey: unique symbol;
+
 export class NodePackageImporter {
+  /** Used to distinguish this type from any arbitrary object. */
+  private readonly [nodePackageImporterKey]: true;
+
   constructor(entryPointDirectory?: string);
 }
 ```
@@ -302,9 +307,11 @@ resolution algorithm. If not set, the default value is the parent directory of
 Defaults to undefined.
 
 ```ts
+import {NodePackageImporter as BaseNodePackageImporter} from '../spec/js-api/importer';
+
 declare module '../spec/js-api/legacy/options' {
   export interface LegacySharedOptions<sync extends 'sync' | 'async'> {
-    pkgImporter?: NodePackageImporter;
+    pkgImporter?: BaseNodePackageImporter;
   }
 }
 ```

js-api-doc/importer.d.ts

@@ -308,6 +308,8 @@ export interface Importer<sync extends 'sync' | 'async' = 'sync' | 'async'> {
   nonCanonicalScheme?: string | string[];
 }
 
+declare const nodePackageImporterKey: unique symbol;
+
 /**
  * The built-in Node.js package importer. This loads pkg: URLs from node_modules
  * according to the standard Node.js resolution algorithm.
@@ -417,6 +419,9 @@ export interface Importer<sync extends 'sync' | 'async' = 'sync' | 'async'> {
  * @category Importer
  */
 export class NodePackageImporter {
+  /** Used to distinguish this type from any arbitrary object. */
+  private readonly [nodePackageImporterKey]: true;
+
   /**
    * The NodePackageImporter has an optional `entryPointDirectory` option, which
    * is the directory where the Node Package Importer should start when

spec/js-api/importer.d.ts.md

@@ -165,7 +165,12 @@ nonCanonicalScheme?: string | string[];
 ### `NodePackageImporter`
 
 ```ts
+declare const nodePackageImporterKey: unique symbol;
+
 export class NodePackageImporter {
+  /** Used to distinguish this type from any arbitrary object. */
+  private readonly [nodePackageImporterKey]: true;
+
   constructor(entryPointDirectory?: string);
 }
 ```